I can confirm that flashing LineageOS->Magisk->OpenGApps works and doesn’t break AVB. However, two of the apps I’d like to use (one banking, one government) have other ways to detect Magisk and won’t work (even with MagiskHide etc.). This is a known issue but it doesn’t look like there’s going to be a quick fix: https://github.com/topjohnwu/Magisk/issues/2406
So I’m currently running just LineageOS+OpenGApps (without Magisk, but having run adb disable-verity and adb reboot 'dm-verity enforcing' after installing OpenGApps in TWRP). That works well for me, except that the same two apps aren’t working (unsurprisingly). Haven’t tested the gov app, but the banking works in plain LineageOS (without OpenGApps/Magisk), so I’m hopeful that building my own ROM with GApps pre-installed will give me what I want.